Method for monitoring bronchoscopic-based microwave ablation and related system

ABSTRACT

A novel monitoring method evaluates tissue ablation progress. An antenna in a distal portion of an ablation applicator sends and receives electrical information from the target tissue during ablation. The information is used to determine ablation progress. A related ablation monitoring system includes a power monitor and processor operable to evaluate ablation progress based on reflected electrical properties during the ablation. The invention has particular benefits when used in endoscopic-based microwave ablation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a United States National Stage Application filedunder 35 U.S.C 371 of PCT Patent Application Serial No.PCT/US2018/015588, filed Jan. 26, 2018, which claims the benefit ofpriority to U.S. Provisional Patent Application No. 62/450,916, filedJan. 26, 2017.

TECHNICAL FIELD

The present invention relates to electrosurgical devices operable todeliver microwave energy of sufficient intensity to cause targetedablation of tissue located within a human or animal body.

BACKGROUND ART

Microwave ablation (MWA) is one of several energy modalities in clinicaluse for thermal treatment of cancer. The goal of thermal ablation isheating of the target tissue to toxic temperatures, leading to celldeath by coagulation necrosis. Thomas Ryan P., “Microwave Ablation forCancer: Physics, Performance, Innovation, and the Future,” inImage-Guided Cancer therapy, New York: Springer Science+Business Media,2013. MWA is a minimally invasive procedure which can be used forunresectable tumors, or for patients who have complicated medicalconditions that would prevent chemotherapy, radiotherapy, or traditionalsurgery. MWA procedures are typically performed with image guidance suchas ultrasound or computed tomography (CT) to identify the disease,position the applicator, and confirm adequate treatment. G. Deshazer, D.Merck, M. Hagmann, D. E. Dupuy, and P. Prakash, “Physical modeling ofmicrowave ablation zone clinical margin variance,” Med. Phys., vol. 43,no. 4, p. 1764, April 2016. MWA has shown less complication rates thansurgical resection and therefore makes it a preferred option forhigh-risk patients not suited for more physically demanding or invasivetreatments. Deshazer et al.

With reference to FIG. 1 , MWA is typically performed by inserting arigid, needle-like antenna applicator 10 into the target tumor 30 of apatient's lung 24 and applying energy supplied from a microwavegenerator. Common frequencies range from 915 MHz or 2.45 GHz, althoughsystems operating at other frequencies are under consideration. S.Curto, M. Taj-Eldin, D. Fairchild, and P. Prakash, “Microwave ablationat 915 MHz vs 2.45 GHz: A theoretical and experimental investigation,”Med. Phys., vol. 42, no. 11, pp. 6152-6161, November 2015. Ideally, MWAprocedures would ablate the tumor with an additional 5-10 mm marginalong the entire tumor boundary 50 to account for any additional cancercells undetected by imaging. See Deshazer et al. Heat is produced whenthe rapidly oscillating radiated electric field induces rotation ofpolar molecules in tissue (such as water) which attempt to align withthe orientation of the applied field. Thermal damage induced in tissueis a function of the transient temperature profile during heating;coagulation necrosis occurs as tissue temperatures exceed ˜55° C. W. C.Dewey, “Arrhenius relationships from the molecule and cell to theclinic,” Int. J. Hyperthermia, vol. 25, no. 1, pp. 3-20, January 2009.The resulting size and shape (volume) of the treated area is determinedby the antenna's radiating pattern, local heat conduction, and heatlosses due to blood perfusion.

Although currently available percutaneous microwave ablation (MWA)systems for treating lung lesions have demonstrated improved local tumorcontrol over other ablation modalities such as laser and radiofrequencyablation (e.g., 88% for MWA compared to 68% and 69% for laser andradiofrequency, respectively T. J. Vogl, R. Eckert, N. N. N. Naguib, M.Beeres, T. Gruber-Rouh, and N.-E. A. Nour-Eldin, “Thermal Ablation ofColorectal Lung Metastases: Retrospective Comparison Among Laser-InducedThermotherapy, Radiofrequency Ablation, and Microwave Ablation,” Am. J.Roentgenol., pp. 1-10, September 2016.), percutaneous ablation comeswith a high associated risk of disrupting the pleural membrane 25.Disrupting the pleural membrane can result in a pneumothorax: a veryundesirable if not deadly complication. It follows that the range oflung tumors that can be accessed with a percutaneous approach arefundamentally limited by the location of the tumors and surroundinganatomy (heart, large blood vessels, diaphragm, ribs). With increaseddetection of peripheral nodules through low-dose CT screening, thenumber of patients with localized disease that can be treated with aminimally-invasive approach is expected to increase substantially. K.Harris, J. Puchalski, and D. Sterman, “Recent Advances in BronchoscopicTreatment of Peripheral Lung Cancers,” Chest, June 2016.

Thermal ablation of lung targets via a bronchoscopic approach has beenproposed as a minimally invasive means for treatment of early-stagetumors. R. Eberhardt, N. Kahn, and F. J. F. Herth, “‘Heat and destroy’:bronchoscopic-guided therapy of peripheral lung lesions,” Respir. Int.Rev. Thorac. Dis., vol. 79, no. 4, pp. 265-273, 2010. Advances inbronchoscopic guidance and navigation techniques are expected toincrease the ability to deliver applicators to targeted tumors viabronchoscopes. D. H. Sterman et al., “High yield of bronchoscopictransparenchymal nodule access real-time image-guided sampling in anovel model of small pulmonary nodules in canines,” Chest, vol. 147, no.3, pp. 700-707, March 2015.

A challenge with MWA devices, however, is to work within the narrowerand longer working lumens of bronchoscope. Bronchoscopic devices must be<2 mm in diameter to fit within working channels of available scopes,and ˜1.5 m long to access targets in the peripheral lung.

Currently available percutaneous MWA devices range in size between 17 G(˜1.5 mm) to 13 G (˜2.4 mm) R. C. Ward, T. T. Healey, and D. E. Dupuy,“Microwave ablation devices for interventional oncology,” Expert Rev.Med. Devices, vol. 10, no. 2, pp. 225-238, March 2013; percutaneous MWAapplicators are typically ˜15-30 cm in length. Smaller diametersnecessitate the use of smaller and longer cables, which yield increasedheating due to electromagnetic attenuation within coaxial cables. Forexample, UT-34 cable at 1.0 GHz has an attenuation coefficient of 1.58dB/m. Considering 60 W applied power, a 20 cm cable for a percutaneousapplicator will yield 2.2 W loss within the cable, compared to 14.3 W ina 1.5 m cable for a bronchoscopic applicator. This added cable heatingundesirably risks thermal damage to the applicator, bronchoscope, andnon-targeted tissue proximal to the applicator's active length.

Microwave ablation assemblies and techniques for heating and destroyingtumor cells within a body are described in various patents, some ofwhich describe use of ablation using flexible elongate members orcatheters. Examples of these assemblies and techniques are described inU.S. Patent Application Publication Nos. 2017/0265940, 2016/0095657,2014/0259641, and 2014/0276739.

Bronchoscopic and endoscopic delivery of microwaves is challengingbecause of the considerable attenuation within thin coaxial cables. Adisadvantage is significant energy losses in connecting cables which mayneed to be compensated for by cooling. See Thomas, Ryan P. The microwavelosses within the cables reduces energy delivered to tissue and leads towaste heating along the applicator that may result in unintended tissueheating, damage to the bronchoscopes/endoscopes, and/or degradation ofdevice performance. Moreover, due to the size constraints ofbronchoscope/endoscope working channels, conventional approaches formitigating microwave ablation zone length (e.g. baluns or triaxialelements) which increase device diameter, are not acceptable.Additionally, there are limited means currently available for monitoringthe progress of microwave ablation.

Accordingly, there is still a need to address the above mentionedchallenges associated with microwave ablation.

SUMMARY OF THE INVENTION

A novel monitoring method evaluates tissue ablation progress. Inembodiments, an antenna in a distal portion of an ablation applicatorsends and receives electrical information from the target tissue duringablation. The information is used to determine ablation progress.

In embodiments, the monitoring comprises establishing a previouswaveform signature of the electrical property over a first discrete timeperiod, detecting a present waveform signature over a second discretetime period, and comparing the baseline waveform signature to thecurrent waveform signature.

In embodiments, the comparing step comprises comparing a baselineenvelope corresponding to the baseline waveform signature and a presentenvelope corresponding to the current waveform signature.

In embodiments, the electrical property is one selected from the groupconsisting of reflected power and impedance.

In embodiments, an ablation monitoring system includes a power monitorand processor operable to evaluate ablation progress based on reflectedelectrical properties during the ablation.

The invention has particular advantages when used for endoscopicmicrowave ablation procedures. In embodiments, a microwave ablationmonitoring system includes techniques for monitoring ablation progressbased on transient measurements of antenna reflection coefficient.

These advantages as well as other objects and advantages of the presentinvention will become apparent from the detailed description to follow,together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a percutaneous microwave ablationprocedure;

FIG. 2 is an illustration of a microwave ablation catheter in accordancewith an embodiment of the invention;

FIG. 3A is an axial sectional view of the distal section of the cathetershown in FIG. 2 according to an embodiment of the invention;

FIG. 3B is an axial sectional view of the distal section of the cathetershown in FIG. 2 according to another embodiment of the invention;

FIG. 4A is a cross sectional view of the catheter shown in FIG. 3A takenalong line 4A-4A;

FIG. 4B is a cross sectional view of the catheter shown in FIG. 3A takenalong line 4B-4B;

FIG. 5 is a schematic diagram of a microwave ablation system inaccordance with an embodiment of the invention;

FIGS. 6A-6B are illustrations of bronchoscopic microwave ablationprocedures corresponding to emitting radiation in different directions;

FIG. 7 is a flow diagram of a method for monitoring microwave ablationof a target tissue;

FIG. 8 is a graphical representation of the simulated antenna reflectioncoefficient for a MWA applicator in lung tissue;

FIG. 9 is a graphical representation of the simulated SAR profile of aflexible MWA applicator;

FIG. 10 is a graphical representation of the simulated ablation zonetemperature profile;

FIG. 11 is an illustration of a prototype MWA applicator;

FIG. 12A is a partial illustration of the prototype MWA shown in FIG. 11inserted into the proximal end of a working channel of a bronchoscope;

FIG. 12B is a partial illustration of the prototype MWA shown in FIG. 11extending from the distal end of the working channel of thebronchoscope;

FIGS. 13A-13B are illustrations of two ablation results in ex vivoporcine loin muscle;

FIGS. 14A-15B are illustrations of various ablation results in porcinelung;

FIGS. 16-17 are a graphical representation of radial temperature vs.time during a lung ablation at various powers;

FIGS. 18-19 are in vivo canine lung ablation 10-day CT images;

FIG. 20 is a graphical representation of reflected power vs. time for aMWA procedure; and

FIG. 21 is an enlarged-scale view of the graphical representation shownin FIG. 20 .

DISCLOSURE OF THE INVENTION

Before the present invention is described in detail, it is to beunderstood that this invention is not limited to particular variationsset forth herein as various changes or modifications may be made to theinvention described and equivalents may be substituted without departingfrom the spirit and scope of the invention. As will be apparent to thoseof skill in the art upon reading this disclosure, each of the individualembodiments described and illustrated herein has discrete components andfeatures which may be readily separated from or combined with thefeatures of any of the other several embodiments without departing fromthe scope or spirit of the present invention. In addition, manymodifications may be made to adapt a particular situation, material,composition of matter, process, process act(s) or step(s) to theobjective(s), spirit or scope of the present invention. All suchmodifications are intended to be within the scope of the claims madeherein.

Methods recited herein may be carried out in any order of the recitedevents which is logically possible, as well as the recited order ofevents. Furthermore, where a range of values is provided, it isunderstood that every intervening value, between the upper and lowerlimit of that range and any other stated or intervening value in thatstated range is encompassed within the invention. Also, it iscontemplated that any optional feature of the inventive variationsdescribed may be set forth and claimed independently, or in combinationwith any one or more of the features described herein.

All existing subject matter mentioned herein (e.g., publications,patents, patent applications and hardware) is incorporated by referenceherein in its entirety except insofar as the subject matter may conflictwith that of the present invention (in which case what is present hereinshall prevail).

Reference to a singular item, includes the possibility that there areplural of the same items present. More specifically, as used herein andin the appended claims, the singular forms “a,” “an,” “said” and “the”include plural referents unless the context clearly dictates otherwise.It is further noted that the claims may be drafted to exclude anyoptional element. As such, this statement is intended to serve asantecedent basis for use of such exclusive terminology as “solely,”“only” and the like in connection with the recitation of claim elements,or use of a “negative” limitation.

By the terminology “endoscopic applications” it is meant to include awide range endoscope-type applications including but not limited tobronchoscopic-type applications. Also by the terminology “applicators”,it is meant to include a wide range of energy emitting devices includingbut not limited to microwave ablation catheters, implements, wands, andrigid probes such as the probes used in a percutaneous approach. It isalso to be appreciated that unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

Described herein are endoscopic-guided microwave ablation devices,systems and methods that enable treatment of central targets, includingbut not limited to targets that are otherwise inaccessible via apercutaneous approach.

FIG. 2 illustrates a flexible microwave ablation (MWA) catheter 200having a proximal handle-like section 210 and an elongate tubular shapedbody 220, and a distal tip 240.

The length of the catheter may vary. In certain embodiments, themicrowave ablation catheter has an insertable length (a length capableof insertion within the patient's body) of at least 0.5 m, at least 0.75m, at least 1.0 m, or at least 1.25 m. In particular embodiments, thecatheter length ranges from about 1 to about 2 m, from about 1.25 m toabout 1.75 m, or about 1.5 m.

The proximal section 210 is shown having a first hub 212 and second hub250 to provide access to the tubular shaft as described further herein.In embodiments, a valve 216 is connected in line with tube 214 to supplya coolant to the catheter 200. An electrical connector 218 is shownextending from the proximal end of the handle. The electrical connector218 can be coupled to a power supply to provide the microwaves asdescribed further herein. It is also to be understood that theconfiguration of the proximal section 210 can vary and the invention canincorporate more or less hubs, tubes, valves, and connectors into thehandle. In embodiments, the proximal section has a slide-hammer orpistol-like shape to ergonomically accommodate the physician during abronchoscopic procedure.

FIG. 3A is an enlarged partial axial section of the distal section 230of the catheter 200 shown in FIG. 2 according to an embodiment of theinvention. The catheter 230 is shown including a generally tubular bodyformed by a multi-lumen section 231 coupled to a single lumen segment240. An antenna 254 is shown extending a distance L₁ from a transmissionline 250, both of which are disposed within the catheter tubular body230. The tubular body is shown extending a distance L₂ from the end ofthe antenna. The distances L₁ and L₂ can be selected based on theelectromagnetic wavelength, and may vary. It is noted that theelectromagnetic wavelength is a function of the system operatingfrequency, and the electrical properties of the materials surroundingthe antenna, including the coolant, dielectric encapsulating plug,catheter, and/or lung tissue. For a system operating at a frequency ofabout 2.45 GHz, for example, the distance L₁ ranges from 4-24 mm and thedistance L₂ ranges from 0.5-3 mm.

The multi-lumen section 231 is a dual lumen or side by side lumencatheter having a wall 236 for separating the lumens. Wall extends thelength of the catheter and is shown terminating a distance L₃ from thetip of the antenna 256. In embodiments, the distance L₃ ranges from 7-28mm.

The two lumens shown in FIG. 3A include a liquid outflow passageway 232and a liquid inflow passageway 234 that can extend the length of thecatheter body. A coolant 350 is circulated through the liquid outflowpassageway 232 and liquid inflow passageway 234 serving to cool thecatheter body and reduce collateral damage to tissue during ablation. Anexample of a coolant is water. Although lumens 232 and 234 areidentified as outflow and inflow lumens respectively, the direction ofthe circulating liquid may be reversed. In embodiments, the coolant 350is driven towards the target tissue through lumen 232 and away from thetarget tissue through lumen 234.

FIG. 3A also shows a portion of antenna 254 encapsulated by an insulator310. The insulator is preferably a low-loss dielectric material.Non-limiting examples of low-loss dielectric materials suitable forinsulator include PTFE (Teflon), FEP, ceramics (e.g. Alumina) andepoxies. Microwaves can pass through the insulator, but not electricalcurrent. Additionally, as discussed herein, the insulator desirablymodifies the shape of the radiation pattern by limiting the extent towhich the coolant may travel in relation to the antenna. In embodiments,the insulator is set a threshold distance L₄ from the antenna distal tipsuch that the coolant attenuates only a portion of the microwave energyduring ablation.

Without intending to being bound to theory, the rationale for partial orincomplete-encapsulation of the antenna is that in some embodiments,e.g. monopole type antennas as shown in FIG. 3A, the antenna radiationpattern is relatively long. That is, the radiation pattern extendsproximally along the cable resulting in a longer than desired ablationzone. To restrict the length of the ablation zone, water is circulatedaround the proximal part of the monopole, but the distal tip remainsencapsulated in epoxy 310 (or other low-loss dielectric material). Thus,microwaves traveling proximally are attenuated by the water.

Although there is also some attenuation of the microwaves travelingradially outward from the antenna and into the tissue, we have foundthat the described designs are sufficient for lung applicationsdescribed herein where a desired ablation zone size ranges from 1-10 cm,more preferably from about 2-5 cm, and in some embodiments, about 3-4 cmin diameter.

With reference again to FIG. 3A, a liquid barrier 312 is shown at apredetermined distance L₄ from the antenna distal end 256. The distanceL₄ may vary. In embodiments, the distance L₄ ranges from 2-12 mm.Additionally, in embodiments the distance L₄ is a fraction of the lengthL₁ and may range from ⅛×L₁ to 1×L₁.

Thus, in embodiments, the liquid is transported substantially close orinto contact with the antenna thereby absorbing the microwave energycorresponding to the radiating pattern tail while permitting themicrowave energy to pass to the target tissue. As mentioned above, theliquid barrier 312 may be formed variously. In embodiments, the liquidbarrier is formed using epoxy 310, which also serves to bond theantenna, multi-lumen tube 231, and single lumen tubular element 240together.

Although the catheter tubular body is shown as a combination of tubularsegments, the catheter configuration may vary widely. The tubularcatheter or body may have more or less segments and lumens. The catheterbody may enclose or house a plurality of individual tubes or tubebundles and/or incorporate telescoping-type arrangements. The catheterlumens and segments may be integral with or separately coupled to servethe applicators described herein.

FIG. 4A is an enlarged cross sectional view taken along line 4A-4A ofthe catheter 200 shown in FIG. 3A. Tubular outer member 230 is shownsurrounding transmission line 250. In embodiments, the tubular body hasa wall thickness T₁ ranging from 0.001 to 0.005 in. The diameter oftubular outer member preferably is less than 2 mm so as to beadvanceable through the working lumen of a bronchoscope or endoscope. Inembodiments, the outer diameter D₁ of the tubular member 230 is lessthan 5 mm, less than 3 mm, or less than 2 mm in order to fit within theworking channels of available scopes.

The outer tubular body 230 in FIG. 4A is divided into a liquid inflowlumen 234 and liquid outflow lumen 232 by wall 236. In embodiments, wall236 has a thickness T₂ ranging from 0.001 to 0.005 in. The wall isspaced at a height Hi from the bottom of the tubular body 230 rangingfrom 0.01 to 0.02 in. Each of the liquid outflow lumen 232 and liquidinflow lumen 234 are shown filled with liquid 350.

As stated herein, the liquid serves both as a coolant and microwaveenergy absorber. An exemplary liquid is water. The extent that themicrowave antenna radiating element is surrounded by water can beadjusted to control the amount of microwave energy traveling backwardsand heating non-targeted objects (e.g., airways, scope, blood vessels,the heart). Use of materials other than water (e.g., materials withdifferent dielectric properties) provides another means for adjustingbackwards radiation. Selection of the optimal material(s) and length ofradiating element in contact with the material(s) provides a means forlimiting backward radiation without increasing device diameter. Inembodiments, and as discussed further herein, the coolant attenuates atail end of the radiation pattern.

The transmission line 250 shown in FIG. 4A includes an inner conductor254 and outer conductor 260 separated by a dielectric layer 264. Thetransmission line 250 also has an outer sheath or jacket which may be anelectrically non-conducting material such as for example a plastic. Inembodiments, the diameter D₂ of the transmission line ranges from 1 to1.3 mm. The transmission line extends through the body of the catheterfrom the proximal section to the distal section and serves to transmit amicrowave signal or energy from the power supply to the antenna, asdescribed further herein.

In embodiments, the transmission line is constructed of a thin andflexible coaxial cable, with bend radius suitable for reaching ablationtargets via a bronchoscopic/endoscopic approach. In embodiments, thebend radius is on the order of 2.5 cm, 2.0 cm, 1.5, cm, 1 cm, or less.In embodiments, the outer conductor 260 is a braided electricallyconducting material or filament structure for improved flexibility alongthe length of catheter. In embodiments, the inner conductor 254 is alsoa braided electrically conducting material.

The use of coaxial cables with braided center and outer conductors asdescribed herein considerably enhances flexibility. In addition, the useof a coaxial cable with outer plastic jacket reduces “set”, a phenomenawhere the instrument exiting the working channel of the endoscope takesa path other than the path defined by the endoscope's tip, and improvescooling efficiency. Amongst other things, the plastic jacket adds alayer of thermal insulation and provides a low friction flow surface.

FIG. 4B is an enlarged cross sectional view taken along line 4B-4B ofthe catheter 200 shown in FIG. 3A. Tubular outer member 230 is shownsurrounding antenna 254. An annular space is shown filled with liquid350. The liquid makes direct contact with the antenna, cooling theantenna as well as absorbing energy during activation of the microwaveenergy. As described herein, the shape and pattern of the radiatingenergy is adjusted by setting the liquid barrier 312 closer or fartherfrom the tip of the antenna.

FIG. 5 schematically depicts an exemplary electrosurgical system 500constructed in accordance with one embodiment of the present invention.An electromagnetic power source 510 generates and transmits the desiredmicrowave energy to antenna 30. Electromagnetic power source 510 mayinclude a microwave signal generator, a DC power supply, a poweramplifier (not shown). Power monitor 520 monitor's electrical activityto and from the device 30 such as, for example, reflected powerimpedance. The operation of device 30 and the various components ofequipment utilized by power source 510 may be monitored and controlledby a microprocessor, such as in a personal computer 530, server, or ahandheld device.

FIG. 5 also shows a pump 542 in fluid communication with the MWAapplicator. The pump serves to circulate the liquid through the MWAapplicator. Adjusting the flowrate can be used to affect the cathetertemperature and radiation pattern described further herein. Exemplaryflowrates range from 5-40 ml/min. An example of a pump is a peristalticpump. However, other means as is known to those of skill in the art maybe used to drive the liquid through the MWA applicator includingelectric pumps, syringes, gravity drip feeds, etc.

In embodiments, the frequencies generated by the signal generator aresimilar to those that are associated with the frequencies typically usedto heat water. In embodiments, the frequencies generated range fromabout 800 MHz to 6 GHz, from about 900 MHz to about 5 GHz, or from about1 GHz to about 3 GHz. In preferred embodiments, particularly for devicesused for experimental clinical work, the frequencies generated are 915MHz or 2.45 GHz. Operating at an operating frequency of 2.45 GHz isdesirable because the higher frequency option results in smaller antennaphysical dimensions due to a shorter wavelength. This is helpful inminiaturizing the length of the active portion of the device. In someembodiments, the MWA system is operated from about 5-6 GHz. Furthermore,MWA systems operating at 2.45 GHz produce more spherical/symmetricablation zones than 915 MHz, which is helpful in minimizing the volumeof ablated healthy tissue when targeting small malignancies.

FIG. 5 also shows temperature probes 540. Temperature probes 540 canoptionally be inserted into the tissue 550 along with device 30 so as tomonitor the temperature of tissue being ablated and adjacent to thetissue being ablated. In embodiments, fiber optic (or othernon-metallic) temperature sensors are employed. Or, temperature sensorsmay be incorporated into the devices by use of thermistors orthermocouples placed on an exterior of the shaft of the device or withinthe liquid near the distal tip so as to monitor temperature, and/orchanges in temperature during an ablation.

As described herein, in embodiments, antenna 30 is designed to have animpedance close to that of the transmission line from signal generator(nominally, 50 ohms) at the operating frequency. The impedance presentedby antenna 30 is a function of the dimensions of the antenna as well asthe wavelength at the operating frequency. Because of this impedancematching, the device can be used in methods of treating body tissuesthat are in close proximity to critical structures. See also US PatentPublication No. 2017/0265940 to Prakash et al, herein incorporated byreference in its entirety.

FIG. 6A is an illustration of a bronchoscopic microwave ablationprocedure on a tumor 30 in the lung 24 of a patient. Initially, abronchoscope 620 is advanced through the nose or mouth into the trachea20 and mainstem bronchi 22. A guidance sheath 622 is shown advancedthrough the scope, and towards the target tissue 30. The MWA catheter610 is then advanced from the guidance sheath 622 or bronchoscope 620into the tumor 30.

As described herein, in embodiments, the MWA catheter has a pre-setshape which facilitates the tip of the catheter 610 to push throughparenchymal tissue in the lung 24. The catheter may include tubularlayer or spine elements formed of materials that provide pushabilitysuch as Nitinol or other superelastic materials.

With reference to FIG. 6A, the catheter 610 is shown advancedsubstantially along a central axis of the tumor 30. The emittedmicrowave radiation 50 is omnidirectional. The radiation is emitted inall directions from the distal section of the catheter 610. It isdesirable to achieve such spherical ablation zones because suchpredictable zones aid practitioners in planning applicator positionrelative to the target. In preferred embodiments, the bronchoscopic MWAapplicator 610 is capable of treating tumors up to 20 mm in diameter(perhaps ˜30 mm ablation zones).

It is also desirable to circulate coolant to mitigate the heat createdduring the ablation and to reduce collateral damage to the non-targettissues (e.g., the trachea 20, bronchi 22, blood vessels, and the heart)and to the instrumentation (e.g., the scope 22).

Additionally, as described herein, the coolant flowpath is determined tocirculate coolant along the body of the catheter for cooling purposes,and also to absorb a desired amount of radiation emitted from theantenna, thereby defining or limiting the radiation pattern arising fromthe antenna. Embodiments of the invention include providing a liquidbarrier at a predetermined distance from the antenna tip to allow thecoolant to flow across the antenna or in close proximity to the antennathereby absorbing a desired amount of radiation.

FIG. 6B is another illustration of a bronchoscopic microwave ablationprocedure on a tumor 30 in the lung 24 of a patient. The procedure shownin FIG. 6B is similar to that shown in FIG. 6A except the MWA catheter660 is configured to emit microwave energy directionally 670.

The MWA catheter 660 is configured to emit microwave energy only towardthe targeted tissue 30 with a directional radiation pattern 670. Thephysician or operator of the device may orient the device such thatenergy is emitted substantially toward the target structure and awayfrom the critical tissues that should not be damaged. Examples ofdevices adapted to emit the microwave energy directionally are describedin US Patent Publication No. 2017/0265940 to Prakash et al, hereinincorporated by reference in its entirety.

FIG. 7 . Shows a flow chart of a microwave ablation method 700 inaccordance with an embodiment of the invention.

Step 710 states to advance the scope. In embodiments, a bronchoscope isadvanced into the patient's lung via the mouth or nasal passageways.Examples of scopes include without limitation a bronchoscope, endoscope,colonoscope, etc.

Step 720 states to advance the microwave ablation catheter through thescope. The physician advances the MWA catheter through the working lumenof the scope, or through an optionally placed guidance sheath which hasbeen advanced through the scope, and towards the target. Examples oftargets include without limitation tumors and suspect tissue growths. Inembodiments, the microwave ablation catheter is advanced through acentral axis of the tumor.

Step 730 states to ablate target tissue by emitting microwave energy. Asdescribed herein, microwave power is transmitted to the end of theantenna and emitted therefrom. The radiation pattern can vary. Inembodiments the radiation pattern is cylindrically symmetric about theaxis of the antenna.

Additionally, the radiation pattern may be adjusted by circulatingcoolant in the vicinity of the antenna. In embodiments, the radiationpattern is modified by defining a liquid barrier a predefined distancefrom the end of the antenna. Preferably, the radiation pattern islimited or confined to the ablation zone near the distal section of thecatheter. The proximal region of the radiation (namely, the tail) islimited by the presence of the circulating coolant.

Step 740 states to monitor electrical properties from the ablation step.As described further herein, a power monitor can be used to monitorelectrical properties to and from the antenna. Examples of electricalproperties to monitor include but are not limited to reflected power andimpedance.

Step 750 states to evaluate electrical properties for completion of theablation of the target tissue. As described further herein in connectionwith FIGS. 20-21 , electrical properties such as reflected power orimpedance may be monitored for patterns which are indicative of thetissue being ablated. Without intending to being bound to theory, thereflected power or impedance of lung tissue during ablation has acharacteristic and repeatable signature corresponding to the reductionor elimination of air in the target tissue over a time period. Inembodiments, a baseline or first signature of the reflected power isobtained over a time period, and an actual or current signature of thereflected power is obtained over a time period, and the actual signatureis compared to the baseline signature. In embodiments, when thedifference between the baseline signature and actual signature is lessthan a threshold value, the tissue is considered ablated. In someembodiments, the signal being higher than the threshold indicates acompleted ablation (e.g., the tumor is not involved in respiration andablating the tumor exposes the applicator to the time varying electricalproperties of the surrounding healthy lung tissue).

If the target tissue is not ablated, the ablation is continued asindicated by returning to step 730. In embodiments, the ablation time ortreatment time may be continuous, and range from 1-15 minutes, morepreferably between 2-5 minutes, frequency ranges between at 915 MHz or2.45 GHz, and more preferably between 2.0 and 2.5 GHz.

In addition to continuing the ablation, the electrical propertymeasurements discussed herein may be used to guide the adjustment ofapplied power during the procedure. The power may be adjusted higher orlower or otherwise adjusted based on feedback from the electricalproperty measurements. The processor can be programmed to determine whenand how to adjust the power based on the measured electrical properties.

Finally, step 760 states to stop ablation. The procedure is complete.

EXAMPLES

A prototype device was fabricated and evaluated via computer modelsimulation. Ex vivo porcine tissues were ablated to verify simulationresults and serve as proof-of-concept. Additional in vivo experimentswere conducted in healthy porcine and canine lung tissue.

To assess technical feasibility of delivering MWA via a bronchoscopicapproach, we constructed and tested a water-cooled, coaxial monopoleantenna. The antenna was constructed by stripping away the outer shieldof the coaxial transmission cable and exposing the center conductor. Anantenna length of 14 mm was calculated based on the expected wavelengthof the radiated electromagnetic wave in lung tissue. Although coaxialantennas without a balun/choke are known to yield radiation patternswith a significant tail, the limited space within 2 mm diameterapplicators precluded the use of a balun.

Example 1. Computer Model Simulation

Coupled finite element method (FEM) electromagnetic-heat transfersimulations were used to characterize antenna design specific to lungtissue. The FEM simulations were employed to assess the antennasimpedance matching, radiation pattern, and thermal ablation profile.

Simulations employed tissue properties as detailed in J. Sebek, N.Albin, R. Bortel, B. Natarajan, and P. Prakash, “Sensitivity ofmicrowave ablation models to tissue biophysical properties: A first steptoward probabilistic modeling and treatment planning,” Med. Phys., vol.43, no. 5, p. 2649, May 2016.

FIG. 8 illustrates the simulated antenna reflection (SAR) coefficient,S₁₁. The S₁₁ parameter is a measure of the impedance match between thecoaxial transmission line and the antenna. Smaller (i.e. more negative)S₁₁ values indicate a good impedance match between the transmission lineand the antenna. Power radiated by the antenna is absorbed withinsurrounding tissue. Conversely, larger S₁₁ values indicate increasedreflected power, which would result in reduced power delivered totissue, and increased heating within the coaxial feedline. FIG. 8indicates the evaluated MWA catheter has an optimal SAR (namely, lowest)at a frequency of about 2.5 GHz.

FIG. 9 illustrates a simulated SAR profile 770 of the MWA applicatordescribed above. As shown, the profile has a proximal tail 772 whichtapers to the shaft with distance from the distal tip.

FIG. 10 illustrates a simulated transient temperature profile 780.Electromagnetic simulations were coupled with a heat transfer model tosimulate the transient temperature profile following ablation with 40 Wapplied power for 300 s. The 60° C. isothermalntour line 775, as anestimate of the anticipated ablation zone, is overlaid on the simulatedtemperature profile. This indicates the evaluated MWA catheter causes aspherical ablation zone.

Example 2. Ex Vivo Porcine Muscle

With reference to FIG. 11 , a prototype flexible applicator 800 wasfabricated using a 1.3 m long coaxial microwave transmission cablejacketed by custom extruded multi-lumen tubing as described above. Weused Vestamid tubing. The tubing provides the flow path for coolingwater and a low friction protective outer surface. The applicator 800 isless than 2 mm in total outside diameter. The distal end of the deviceis sealed with an epoxy plug.

A hemostasis valve 830 at the proximal end of the device allowsinsertion of the coaxial cable 832 into the extruded tubing and providesconnection for the circulating water system. Ice water was circulatedwith a peristaltic pump (not shown). The cooling system removes heatcoming from cable attenuation and reflected power to prevent devicedamage and unintended heating of surrounding healthy tissue.

FIG. 12A is an enlarged partial view of the proximal section of abronchoscope 840 and illustrates the flexible MWA applicator 800 shownin FIG. 11 inserted into a hub 848 of a bronchoscope. The MWA applicatoris advanced to marker 812, corresponding to a predetermined distance theapplicator tip 820 shall be extended from the bronchoscope end 842.

FIG. 12B is an enlarged partial view of the distal section 842 of thebronchoscope and illustrates the distal section 820 of the flexible MWAapplicator 800 shown in FIG. 11 extending distally from the workinglumen 844 of the bronchoscope.

Both porcine loin muscle and lung tissues were obtained fresh and keptin sealed bags placed on ice for use in device characterization. Priorto use, the tissue was sectioned into approximately 10 cm³ samples andwarmed to approximately 30° C. in a water bath (while sealed in plasticbags). The MWA applicator and four fiber optic temperature sensors wereinserted 6 cm into the tissue using a plexi-glass template which keptthe sensors spaced 5, 10, 15, and 20 mm radially from the applicator.

A HP 8665B signal generator and RFcore RCA0527H49A microwave amplifierwere used to supply the 2.45 GHz signal to the applicator. Forward andreflected power were monitored during the experiments using a BirdTechnologies 7022 statistical power meter.

Ablation zones created with the flexible MWA applicator were firstevaluated within the ex vivo pork loin muscle tissue. Three ablationswere performed and the results are summarized in Table 1 below. FIGS.13A-13B show representative ablation zones when the tissue is sectionedalong the axis of the tissue.

TABLE 1 Ablation results in porcine muscle Power Duration HeightDiameter Axial (W) (min) (mm) (mm) Ratio 40 4 27 19 0.70 40 4 33 24 0.7340 4 27 17 0.63 Mean 29 20 0.69 Std. dev. 3.5 3.6 0.05

Example 3. Ex Vivo Porcine Lung

Next, with the applicator inserted into lung tissue, an appropriateimpedance match was confirmed when the antenna S₁₁ of −21.8 dB wasmeasured at 2.45 GHz using an HP 8753D vector network analyzer. Examplesof desirable impedance matches range from ˜8 to −25 dB.

The first ex vivo lung ablation performed at 40 W as measured by thepower meter did not produce any visible ablation region. This may havebeen due to applicator positioning within a large airway, describedfurther herein. Results from additional ablations performed at 60 and 80W are given in Table 2 below.

TABLE 2 Ablation results in porcine lung Power Duration Height DiameterAxial (W) (min) (mm) (mm) Ratio 40 4 — — — 60 5 31 13 0.42 60 5 32 100.31 60 5 — — — 80 5 32 13 0.41 80 5 17 15 0.88

FIGS. 14A-14B show results for the two 60 W, 5 min, porcine lungablations.

FIGS. 15A-15B show results for the two 80 W, 5 min, porcine lungablations.

We also noted the third ablation performed at 60 W exhibited a barelyvisible ablation zone; its boundary was so faint and diffuse that itcould not be accurately measured. The experimentally observed ex vivoablation zones were anticipated to be smaller than simulated ablationzones because effects such as power lost to cable attenuation and tissuecontraction, amongst other things, were not modeled. C. L. Brace, T. A.Diaz, J. L. Hinshaw, and F. T. Lee, “Tissue contraction caused byradiofrequency and microwave ablation: a laboratory study in liver andlung,” J. Vasc. Interv. Radiol. JVIR, vol. 21, no. 8, pp. 1280-1286,August 2010.

Collectively, the data and FIGS. 13A, and 13B show the ablation patternsobserved in porcine muscle were in close agreement with simulations(diameter=20 mm, height=29 mm, axial ratio=0.69 in experiments vs.diameter 32 mm, height 42 mm, axial ratio=0.76 from simulation).However, ablation zones observed in lung tissue were variable and not incomplete agreement with the simulated ablation zone shapes/sizes.Without being bound to theory, the variability in the lung tissue arisesdue to its spongy nature. Lung tissue is more heterogeneous than muscledue to the number of large diameter airways. These airways causesignificant electromagnetic reflection which can affect the consistencyof results and the visible appearance of experimental ablation zone.Lung is also much more elastic than other tissues which can cause sampletissue deformation or inaccurate applicator/probe placement duringexperiments. Further characterizing and predicting ablation size andshape in ex vivo lung tissue, including a suitable tumor mimic, is thesubject of ongoing work. T. Kawai et al., “Creation of a tumor-mimicmodel using a muscle paste for radiofrequency ablation of the lung,”Cardiovasc. Intervent. Radiol., vol. 32, no. 2, pp. 296-302, March 2009.

Despite the variability in ablating the lung tissue, we observed thatthe application of microwave energy raised lung tissue to ablativetemperatures. As stated above, we measured temperatures using fourprobes spaced apart from one another. The temperatures measured at eachprobe are set forth in FIGS. 16-17 . Particularly FIG. 16 shows theapplication of 60 W will raise lung tissue 1 cm radially from theapplicator to temperatures near 60° C. in about 220 seconds. FIG. 17shows the application of 80 W will cause the same effect in about halfthe time.

Example 4 In Vivo Canine Ablation

Another test of our MWA catheter was performed in vivo on a caninespecimen. All ablations were performed under an experimental protocolapproved by the local institutional animal care and use committee.Following induction of anesthesia, MWA applicators as described abovewere advanced to the target tissue via the working channel of abronchoscope. Four ablations were performed, two in each lung, each at60 W for 5 minutes.

Return loss measurements were recorded by the power meter during theprocedures and ranged from −12.3 to −17.2 dB.

The primary objectives of this study were to verify proof-of-concept,safety, and containment of the ablation sites in the lung. Followingablation procedures, the animals were recovered from anesthesia andsurvived for 10 days. CT images were obtained at two days and ten dayspost procedure.

FIGS. 18-19 show the in vivo canine lung ablation 10-day CT images.Based on analysis of the CT data the estimated size of the ablationzones 860, 870 are given in Table 3 below.

TABLE 3 In vivo ablation results 2-day CT 10-day CT Power Duration MinorMajor Minor Major (W) (min) (mm) (mm) (mm) (mm) 60 5 18 37 5 17 60 5 1622 4 19 60 5 17 42 5 20 60 5 13 50 4 25 Mean 16 37.75 4.5 20.25 Std. 2.211.8 0.6 3.4 dev.

The results of the in-vivo study showed that our prototype flexible MWAapplicator was able to generate contained ablation zone in the lungtissue safely. No adverse effects were observed after the ablations werecompleted. Microwave ablation of lung tumors with a flexiblebronchoscopic device offers a minimally invasive procedure andalternative to non-surgical candidates. We were able to overcome thechallenges in the design and construction with significant engineeringtradeoffs.

Our pilot in vivo experiment demonstrated the safety and containment ofmicrowave energy within living lung tissue. Though embodiments of thepresent invention are described in connection with treatment of thelung, the present invention is not so limited. The invention is alsointended for use in other minimally invasive endoscopic procedures.

Example 5. In Vivo Canine Monitoring

As discussed above, lung tissue involved with respiration will have timevarying dielectric properties due to varying air content over arespiratory cycle. As lung tissue coagulates during ablation, it plays areduced role in respiration, and consequently dielectric propertyvariations during ablation are less pronounced. These changes can bedetected with reflected power measurements at the ablation device inputport. It has been demonstrated experimentally (with in vivo experiments,n=4 ablations in canine lung) that the time varying oscillations duringablation decreases with time, thus providing a means to assess thephysical state of tissue in proximity to the device. Monitoring ablationprogress based on measurements of transient antenna reflectioncoefficient may be applied to endoscopic treatment of targets besideslung tissue, such as the pancreas, where there are cyclic variations intissue physical properties/state (e.g. ablations of target within or inproximity to blood vessels where there are cyclic changes in flow).

Benchtop experimental results in excised animal tissue demonstratedsuitable ablation zone size, and control of radiation along the deviceaxis.

A custom dual-lumen catheter was extruded with lumen areas calculated soas to provide balanced flow in each lumen when the coaxial cable wasinserted (such as the catheter lumen shown in FIG. 4A). The dual lumencatheter was made of polyamide such as the Vestamid® brand tubingmanufactured by Vestamid (Essen, Germany) so as to ensure thermalstability of the catheter at high temperatures. In benchtop experimentsconducted with 45 W applied power, catheter temperature was measured tobe ˜34.2 C (mean value from 29 experiments) during ablations, comparedto ˜40.9 C (mean value from 24 experiments) for 60 W experiments, whenemploying the same coolant temperature/flow settings. This data reflectsthat for a given coolant flow, the catheter temperature was controlledbased on the power settings. We also conclude that for a given powerlevel, catheter temperature would predictably be controlled based on thecoolant flow settings.

In vivo experimental results in a canine model show the technicalfeasibility of creating and monitoring progress of local ablation zonesin the lungs via a bronchoscopic approach. During in vivo microwaveablation experiments, with reference to FIGS. 20-21 , reflected powermeasurements 910 with ablation antenna varied cyclically, following therespiratory cycle. The amplitude of the variation in the reflected power910 diminished over the course of the ablations, suggesting remodelingof lung tissue during heating, and that ablated tissue has a reducedrole in respiration. As such, electrical property variations in ablatedtissue are diminished. This decrease in amplitude in the reflected powercan be utilized to monitor ablation progress. Based on these results, achange in ˜60-80% of the amplitude of the cyclic variation in reflectedpower can be used as an indicator of ablation endpoint 940.

Thus, at the beginning of treatment, a “baseline” amplitude of reflectedpower can be established across one or more breathing cycles. Astreatment progresses, the amplitude of reflected power can be expectedto diminish as the tissue becomes remodeled and has a decreased role inthe respiratory cycle. Once the amplitude of reflected power isdetermined to have decreased by at least 50%, at least 60%, at least 70%or at least 80% over one or more respiratory cycles, the localizedhyperthermic treatment can be discontinued. In certain embodiments, thetime for delivering microwaves to the target tissue until the requireddecrease in amplitude of reflected power is observed can be from about75 to about 350 seconds, from about 125 to about 325 seconds, or fromabout 150 to about 300 seconds, depending upon antenna power setting.

With reference to FIG. 21 , line 910 shows variations in reflected powermeasured during the canine lung ablation procedure in vivo. The upperand lower envelopes of the reflected power trace are marked with dashedlines 922, 924 respectively. The “average” value of the reflected powertrace is marked by line 930. As described above, these values, togetherwith the period between successive maxima (or minima) 942, can provideinformation to characterize the pattern or signature of reflected powerchanges.

In embodiments, the applied power is adjusted based on the patterns orsignature of the reflected power changes. A previous waveform signatureof the electrical property over a first discrete time period isestablished, and a present waveform signature over a second discretetime period is detected, and the baseline waveform signature is comparedto the current waveform signature. The comparing step can comprisecomparing a baseline envelope corresponding to the baseline waveformsignature and a present envelope corresponding to the current waveformsignature. When the difference between the first and second envelope iswithin a threshold amount, such as less than 70% of the differencebetween the first and second envelope at the onset of the ablation, thenthe ablation may be considered complete.

Alternative Embodiments

Other modifications and variations can be made to the disclosedembodiments without departing from the subject invention.

For example, embodiments of the invention include other types ofantennas including but not limited to: dipole, helical, slot, multipleslot, and monopole type antennas.

An example of a dipole-type antenna is shown in FIG. 3B. The antenna 320may be fully or partially encapsulated by the epoxy 310 to prohibit theliquid from attenuating the microwaves intended to travel to the targettissue. In other embodiments, the antenna is not encapsulated.

The tip 320 is spaced from the transmission line a distance M₁. Thedistance M₁ can be based on the electromagnetic wavelength, which is afunction of frequency and the electrical properties of surroundingmaterials. At a frequency of 2.45 GHz, for example, M₁ can range from4-14 mm. The liquid barrier 312 is spaced a distance M₂ from the tip ofthe antenna 320 and the tubular body portion 240 extends a distance M₃beyond or distal to the tip of the antenna 320 thereby fullyencapsulating the antenna. In embodiments, M₂ can range from 7-14 mm andM₃ can range from 0.5-3 mm. Otherwise, the catheter shown in FIG. 3Boperates similar to the catheter shown in FIG. 3A, discussed above.

Embodiments of the invention can optionally include a spine element tocompensate for “set” of the applicator, as well as to increasepushability of the applicator through the narrow passageways of thescope and patient. The spine element such as a nitinol member (or othersuitable material) may be inserted in one lumen of the catheter. Theconfiguration of the spine element may vary. Exemplary spine elementsinclude but are not limited to a wire, tube, layer, or braidedarrangement.

The distal tip 240 of the applicator is generally shown having anatraumatic shape, however, the invention is not so limited. The distaltip 240 of the applicators herein may be sharp, pointed, beveled,rounded, or tapered to facilitate tissue dissection and penetration.

In embodiments the antenna is not encapsulated whatsoever by the epoxyor liquid barrier described herein. Embodiments of the invention includefully cooled antenna designs.

Embodiments of the invention can optionally be used in connection withsensing or imaging equipment configured to give real-time feedback tothe physician conducting a procedure. In embodiments, the novel flexiblemicrowave applicator is integrated with a bronchoscopic imaging andsoftware guidance platform to expand the use of the MWA as a treatmentoption for small (<2 cm) pulmonary tumors. This would allow physiciansan even less invasive, immediate treatment option for lung tumorsidentified within the scope of current medical procedures, improveapplicator placement accuracy and may increase efficacy while minimizingthe risk of procedural complications.

In embodiments, the sensing or imaging equipment can give the physicianinformation regarding the ablation boundary associated with the use ofthe device. If the ablation boundary does not extend to the edge of thedesired target, the physician can reposition or rotate the device totreat the full extent of tissue in between the desired margins. Forexample, the device can be fabricated from MRI-compatible materials foruse under MRI guidance. Such devices do not generate a visible imagingartifact when introduced into an MRI bore. Use of a device with an MRIoffers the benefit of real-time volumetric temperature imaging forfeedback controlled procedures. For instance, when targeting structuresin very close proximity to several critical structures, MRI temperatureimaging could be used to assess when the treatment boundary extended tothe edge of the desired target, and then guide rotation of the device totarget tissue in another direction

Embodiments of the invention can optionally be used with a wide range ofinstruments including but not limited to a bronchoscope, endoscope,colonoscope, etc. The devices described herein can be applied to targettissues in regions that can be accessed percutaneously, endoluminally(e.g., bronchii, urethra, rectum, stomach, esophagus) or endovascularly(e.g., renal nerves). The device may also be used for moderate heatingof tissues (e.g., between about 41 and 44 degrees Celsius) as anadjuvant to radiation and or chemotherapy for treatment of selectcancers.

Other modifications and variations can be made to the disclosedembodiments without departing from the subject invention.

The invention claimed is:
 1. A method for monitoring ablating a targettissue in a lung of a patient comprising the steps of: advancing abronchoscope through the mouth or nose of the patient and into the lung;advancing a microwave ablation catheter through a working lumen of thebronchoscope and into or adjacent to the target tissue; ablating thetarget tissue by emitting microwave energy from the microwave ablationcatheter to the target tissue at an ablative power to cause ablation ofthe target tissue; and monitoring the step of ablating for determiningwhether the target tissue has been ablated; wherein the monitoring isperformed by monitoring amplitude of the ablative power reflected backto the microwave ablation catheter from the target tissue during theablating step; and wherein the monitoring comprises establishing aprevious waveform signature of the amplitude of the ablative powerreflected back to the microwave ablation catheter over a first discretetime period, detecting a current waveform signature over a seconddiscrete time period, and comparing the previous waveform signature tothe current waveform signature.
 2. The method of claim 1, wherein themonitoring is performed by sensing for the presence of air within thetarget tissue during ablation.
 3. The method of claim 1, wherein theprevious waveform signature is a baseline envelope that envelops acyclic waveform corresponding to the amplitude of the power beingreflected back to the microwave ablation catheter over the firstdiscrete time period, and wherein the current waveform signature is acurrent envelope that envelops a cyclic waveform corresponding to theamplitude of the power being reflected back to the microwave ablationcatheter over the second discrete time period, and wherein the comparingcomprises comparing the baseline envelope corresponding to the previouswaveform signature and the current envelope corresponding to the currentwaveform signature.
 4. The method of claim 1, wherein the step ofablating the target tissue comprises ablating a tumor.
 5. The method ofclaim 1, wherein the step of ablating the tissue by emitting microwaveenergy is performed at a frequency in the range from 0.3-6 GHz.
 6. Themethod of claim 5, further comprising monitoring temperature.
 7. Themethod of claim 6, further comprising circulating a liquid through themicrowave catheter, and adjusting a flowrate of the liquid based on thetemperature from the monitoring step.
 8. The method of claim 1, furthercomprising halting the ablation if the amplitude decreases relative to abaseline amplitude by a predetermined threshold amount.
 9. The method ofclaim 8, wherein the threshold amount is greater or equal to 70%.
 10. Amethod for monitoring ablating a target tissue in an organ of a patientcomprising the steps of: advancing an endoscope through a naturalorifice to the target tissue in the organ; advancing an ablationcatheter through a working lumen of the endoscope and into or adjacentto the target tissue; ablating the target tissue by emitting energy fromthe ablation catheter to the target tissue at an ablative powersufficient to cause ablation of the target tissue; and monitoring thestep of ablating for determining whether the target tissue has beenablated and wherein the monitoring is performed by monitoring amplitudeof the ablative power reflected back to the microwave ablation catheterfrom the target tissue during the ablating; and wherein the monitoringcomprises establishing a previous waveform signature of the amplitude ofthe ablative power reflected back to the microwave ablation catheterover a first discrete time period, detecting a current waveformsignature over a second discrete time period, and comparing the previouswaveform signature to the current waveform signature.
 11. The method ofclaim 10, wherein the advancing an endoscope through a natural orificeto the target tissue in the organ is advancing a bronchoscope throughthe mouth or nose and into the lung.
 12. The method of claim 10, whereinthe comparing comprises comparing a baseline envelope corresponding tothe previous waveform signature and a present envelope corresponding tothe current waveform signature.
 13. An ablation monitoring system formonitoring ablation of a target tissue in a patient comprising: amicrowave power source for generating microwave energy; a microwavepower monitor and a processor; and a microwave ablation cathetercomprising: an elongate tubular body comprising a flexible proximalsection and a distal section; an elongate antenna disposed within thedistal section, and comprising a distal tip; a transmission line inelectrical communication with the power source and extending through thebody to the antenna and for transmitting the microwave energy betweenthe power source and the antenna wherein the power source delivers anamount at an ablative power sufficient to ablate the target tissue; andwherein the antenna, microwave power monitor and processor are operabletogether to monitor ablation progress based on monitoring amplitude ofthe ablative power reflected from the target tissue to the microwaveablation catheter during ablation; wherein the monitoring is performedby establishing a previous waveform signature of the amplitude of theablative power reflected from the target tissue to the microwaveablation catheter over a first discrete time period, detecting a currentwaveform signature over a second discrete time period, and comparing theprevious waveform signature to the current waveform signature.
 14. Thesystem of claim 13, wherein the microwave power monitor and processorare operable to monitor ablation progress by sensing for the presence ofair in the target tissue in which the antenna is placed.
 15. The systemof claim 13, wherein the processor is operable to compare a baselineamplitude envelope corresponding to the previous waveform signature anda current amplitude envelope corresponding to the current waveformsignature.
 16. The system of claim 13, wherein the processor isprogrammed to halt the ablation if the amplitude decreases relative to abaseline amplitude by a predetermined threshold amount.
 17. The systemof claim 16, wherein the threshold amount is greater or equal to 70%.18. An ablation monitoring system for monitoring ablation of a targettissue in a patient comprising: a microwave power source for generatingmicrowave energy; a microwave power monitor and a processor; and amicrowave ablation catheter comprising: an elongate tubular bodycomprising a flexible proximal section and a distal section; an elongateantenna disposed within the distal section, and comprising a distal tip;a transmission line in electrical communication with the power sourceand extending through the body to the antenna and for transmitting themicrowave energy between the power source and the antenna wherein thepower source delivers an amount at an ablative power sufficient toablate the target tissue; and wherein the antenna, microwave powermonitor and processor are operable together to monitor ablation progressbased on monitoring the amplitude of the ablative power reflected fromthe target tissue to the microwave ablation catheter during ablation;wherein the processor is programmed to halt the ablation if theamplitude decreases relative to a baseline amplitude by a predeterminedthreshold amount.
 19. A method for monitoring ablating a target tissuein a lung of a patient comprising the steps of: advancing a bronchoscopethrough the mouth or nose of the patient and into the lung; advancing amicrowave ablation catheter through a working lumen of the bronchoscopeand into or adjacent to the target tissue; ablating the target tissue byemitting microwave energy from the microwave ablation catheter to thetarget tissue at an ablative power to cause ablation of the targettissue; monitoring the step of ablating for determining whether thetarget tissue has been ablated; wherein the monitoring is performed bymonitoring amplitude of the ablative power reflected back to themicrowave ablation catheter from the target tissue during the ablatingstep; and halting the ablation if the amplitude decreases relative to abaseline amplitude by a predetermined threshold amount.